Modern RF circuits often require amplified signals that are produced by splitting an input signal, providing it to a number of amplifier modules and then combining the output of the amplifier modules into a single amplified output signal. Often it would be beneficial to employ a circuit in which the number of amplifier modules can be increased or decreased as conditions demand. This allows initial construction of an operative amplifier circuit with a single amplifier module and subsequent addition of one to three amplifier modules to the circuit as technical needs require additional amplification or as financial constraints allow investment in additional amplifier modules.
Conventionally, power splitting and power combining in RF circuits is performed using Wilkinson type power divider/combiner systems, as generally described in “An N-Way Hybrid Power Divider” by E. J. Wilkinson, IRE Transactions on Microwave Theory and Techniques, Vol. MTT-8, No. 1, January 1960, at pp. 116–118. A deficiency of pure Wilkinson type systems is that they operate efficiently only when all of the branches of the divider/combiner include a properly operating amplifier module. If an amplifier module is missing or inoperative, the Wilkinson type divider or combiner loses efficiency because of the impedance mismatch presented by the inoperative/absent amplifier module.
As a result, there have been a variety of attempts to modify Wilkinson type divider combiners to overcome the difficulties presented by inoperative amplifiers. One such attempt is described in U.S. Pat. No. 5,543,751 to Stedman. Through the use of phasing lines and matching lines, each having a specific length and impedance, the Stedman power combiner is capable of acceptable efficiency if one or more of the installed amplifiers should fail. That is, the power loss if 1, 2 or 3 of the installed amplifiers fails (in a four-amplifier system), does not exceed 11% of the input power. In addition, to operate properly the Stedman device requires that all amplifier positions be populated with an amplifier.
Another attempt to solve the difficulty of inoperative amplifiers is described in U.S. Pat. No. 5,767,755 to Kim. Again, through the use of phasing lines and matching lines, each having a specific length and impedance, the Kim power combiner is capable of operating at acceptable efficiency levels even if one or more of its amplifiers fail. That is, the power loss if 1, 2 or 3 of the installed amplifiers fails, does not exceed 11% of the input power. To operate properly, the Kim device requires that all amplifier positions be populated with an amplifier.
A more complicated method of addressing the difficulty posed by inoperative amplifiers is described in U.S. Pat. No. 4,315,222 to Saleh. The power combiner disclosed in Saleh requires switches and sensing means to maximize performance of the network in case one of the amplifiers fails. When a failed amplifier is sensed, the switches alter the presented impedance to maintain an acceptable level of efficiency. The switches and sensing means of that network increase its complexity and cost.
The power combiner/splitter networks disclosed in Stedman, Kim and Saleh are designed to operate with a specific number of amplifier modules, and further, to provide acceptable levels of signal loss in the case that one or more of the amplifier modules should fail. Furthermore, each is designed to operate properly only when all available amplifier positions are populated with an amplifier. They are not designed to allow population of less than all amplifier positions or to permit subsequent addition of one or more amplifiers.
European Patent EP 0540286B1 to Japan Radio Co., Ltd. discloses a power divider combiner that will operate even when less than all of the amplifier positions is populated. The circuit disclosed in EP 0540286B1, however, must be designed to maximize performance when a selected number of amplifier modules is populated. When more or fewer amplifier positions are populated, performance of the circuit suffers. For example, if the circuit is designed as a four-way circuit that is maximized for performance when three amplifier positions are populated, the return loss when 1, 2 or 4 amplifier positions are populated will be higher than when 3 positions are populated.
What is needed is a four-way power splitter and combiner network that is capable of operating at acceptable efficiency when populated with one, two, three or four amplifier modules and that further provides acceptable signal attenuation and Voltage Standing Wave Ratio (“VSWR”) if the network is populated with less than four amplifier modules.